Semiconductor manufacturing requires precisely measuring various features and parameters. Features which must be precisely measured include, for example, thin films. A thin film of photoresist which is too thick or too thin can dramatically affect the semiconductor manufacturing process. Similarly, a parameter such as the focal length of a lithographic semiconductor manufacturing apparatus dramatically affects the condition and reliability of elements created using the lithographic semiconductor manufacturing apparatus.
Some prior art thin film thickness measuring techniques are based upon UV reflectance spectroscopy. Prior Art FIG. 1 shows a reflection of light, typically shown as rays 10, incident to a homogenous, substantially semitransparent layer 12. Using prior art thin film measuring techniques, once the refractive index, n.sub.1, of substantially semitransparent layer 12, and the refractive index, n.sub.2, of substrate 14 are known, the film thickness t.sub.1 of layer 12 can be determined. The thickness of semitransparent layer 12 is related to measured reflectivity as a function of wavelength, .lambda., via application of the Fresnel reflection formula: ##EQU1##
Thus, prior art thin film thickness measuring methods require knowledge of the refractive index of the thin film being measured and the refractive index of the underlying substrate. The user must also know or calculate the real and imaginary components of indices of refraction. The refractive indices of the thin film and the underlying substrate are commonly determined using a given Cauchy coefficient. Therefore, if the refractive indices are not known, prior art thin film thickness measuring techniques require the user to have known the refractive indices of the thin film and an underlying layer before the thickness of thin film can be calculated. Furthermore, prior art thin film thickness measuring techniques employing reflectance spectroscopy require that the film being measured is not entirely absorptive.
Prior Art techniques such as the "pin-bars" technique also exist for measuring parameters such as the focal length of a lithographic semiconductor manufacturing apparatus. However, prior art focus measuring techniques are subjective. As a result, user error can cause a lithographic semiconductor manufacturing apparatus to be set at a focus which is not the optimal focus. Likewise, differences in perception between different users can cause variances between different lithographic semiconductor manufacturing apparati.
Thus, the need has arisen for a thin film thickness measuring system which does not require the user to know or calculate the refractive indices of the thin film or an underlying layer; and a focal length measuring system which allows for objectively and consistently selecting an optimal focal length.